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Walking Speed at Self-Selected Exercise Pace Is Lower but Energy Cost Higher in

Older Versus Younger Women

Authors: Lynnette M Jones1, Debra L Waters

2, Michael Legge

3

Affiliations: 1School of Physical Education,

2Department of Preventive and Social

Medicine, 3Departments of Biochemistry and Pathology. University of Otago,

Dunedin, New Zealand

Running head: Energy Costs of Walking

Keywords: physical activity, metabolic costs, females, aging

Page 1 of 23 Journal of Physical Activity and Health © Human Kinetics, Inc.

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Abstract

Background: Walking is usually undertaken at a speed that coincides with the lowest

metabolic cost. Aging however, alters the speed-cost relationship, as preferred

walking speeds decrease and energy costs increase. It is unclear to what extent this

relationship is affected when older women undertake walking as an exercise modality.

The aim of this study was to compare the energetic cost of walking at a self-selected

exercise pace for a 30 minute period in older and younger women. Methods: The

energetic cost of walking was assessed using the energy equivalent of oxygen

consumption measured in 18 young (age 25-49 years) and 20 older (age 50-79 years)

women who were asked to walk at their ‘normal’ exercise pace on a motorised

treadmill for 30 minutes duration. Results: The mass-specific net cost of walking

(Cw) was 15% higher and self-selected walking speed was 23% lower in the older

women than in the younger group. When speed was held constant, the Cw was 0.30

(J.kg-1

.m-1

) higher in the older women. Conclusions: Preferred exercise pace incurs a

higher metabolic cost in older women and needs be taken into consideration when

recommending walking as an exercise modality.

Page 2 of 23Journal of Physical Activity and Health © Human Kinetics, Inc.

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Introduction

Walking is a convenient and popular form of aerobic activity and is widely

recommended for health and fitness for individuals of all ages.1, 11, 12, 18, 23

The

associated energy costs of this activity are the result of the need to convert chemical

energy to the mechanical work of walking. Measurement of oxygen consumption

during physical activity allows an assessment of the associated metabolic costs to be

made. The mass-specific net cost of walking (Cw), defined as the energy cost of

moving 1 kg of body mass 1 metre,2, 22

and speed is U-shaped, with an individual’s

walking speed selected to coincide with the lowest metabolic cost.2, 21, 31

This

relationship can be altered by changes in gait patterns as a result of amputation,17

disease,3 loading,

2 obesity,

6, 7 gender,

6 and aging.

8, 15, 16, 19-21, 29

Self-selected walking speeds decrease with age, while the associated energy costs

increase, resulting in an upward shift in the Cw-speed relationship during aging.8, 15, 16,

19-21, 29 The reason for this upward shift in older adults is multifactorial. Determinants

of walking speed and metabolic cost in older individuals that have been proposed

include the effects of age,16, 21

alterations in muscle efficiency,22

step length,27

height,16, 20

body composition,15, 26

and increased cardiorespiratory demands.20, 21

Many studies undertaken to evaluate the metabolic cost of walking at self-selected or

pre-determined walking speeds in older individuals have been undertaken for

relatively short periods of time or over relatively short distances.15, 19-22, 25, 26

Furthermore, no studies to date have focused on the metabolic costs of walking at a

self-selected exercise pace. American College of Sports Medicine (ACSM) physical

activity recommendations for adults suggest moderate intensity exercise be


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undertaken five days per week for 30 minutes per session for adults aged 18-65

years,14

although in adults >65 years this recommendation can be obtained by shorter

duration exercise repeated several times per day.24

It is more difficult to extrapolate

Cw over short periods of time to a continuous ‘exercise-focused’ protocol. Therefore

the aim of this study was to compare the energetic cost of walking at a self-selected

speed for a 30 minute period in older and younger women.

Methods

Forty Caucasian women were recruited by advertisement to participate in this cross-

sectional comparative study, which was approved by the University of Otago Human

Ethics Committee. All women were regular recreational exercisers, ie physical

activity was normally undertaken less than four days or 150 minutes per week.

Participants in the younger (YOU) group comprised women between 25-49 years of

age (n=20), while women aged between 50-79 years comprised the older (OLD)

group (n=20).

Informed written consent was obtained and a medical and exercise screening

questionnaire was completed prior to the first exercise testing session. Exclusion

criteria included a history of heart disease, Type 2 diabetes, orthopaedic problems,

current hormone replacement therapy use, regular use of any medications known to

affect metabolism (eg thyroid drugs), significant weight fluctuations within the six

months (± 5kg) prior to participation in this study, any health problem which may

have interfered with exercise, or an inability to walk continuously for 30 minutes.

Participants were also excluded if they demonstrated a high level of current physical

activity, ie exercised more than four days or 150 minutes per week.


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Anthropometry

Height, weight, waist and hip circumferences were recorded prior to the treadmill

walk. Height was determined without shoes using a free standing stadiometer, while

weight was taken with participants wearing their walking attire, also without shoes,

using electronic scales (Digi, D1-10, Teraoka Ltd, Tokyo, Japan). A standard

fibreglass anthropometric tape was used to measure waist circumference, defined as

the narrowest part between the last rib and the anterior superior iliac spine (ASIS),

and hip circumference, defined as the widest part between the ASIS and the greater

trochanter. Body mass index (BMI - weight (kg)/height (m2)) was used to estimate

adiposity, while waist hip ratios (WHR) were calculated to describe body fat

distribution.

Exercise Protocol

All exercise sessions and maximal exercise testing took place at the School of

Physical Education, University of Otago. The study protocol required the participants

to complete 30 continuous minutes of treadmill walking. Participants were tested

individually and were asked to undertake the exercise sessions at their ‘normal’

exercise pace, ie self-paced exercise. Participants were asked to maintain their normal

diet and activity patterns between the exercise session and the maximal exercise test.

The maximal exercise test was undertaken 48 hours after the exercise session had

been completed to ensure the women chose their own exercise pace during the walk.

Prior experience or perception of higher intensity exercise on the treadmill could have

influenced the choice of exercise pace made by each woman.


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Treadmill walking was undertaken in an exercise physiology laboratory, ambient

temperature 26oC, using a Quinton motorised treadmill (Quinton Instrument

Company, Series 90 Q65, WA, USA) set at 0% gradient. Breath sampling occurred at

rest, for three minutes immediately prior to beginning the treadmill walk, the average

of the final two minutes was used in the analyses. Thereafter the 30-minute exercise

session was divided into five six-minute blocks for sampling purposes. Breath

samples were collected for the final 80 seconds of each block. The Sensormedics

metabolic cart samples every 20 seconds, therefore the initial 20 seconds of data were

discarded and the remaining 60 seconds averaged for analytical purposes. Rating of

perceived exertion (RPE), Borg scale 6-20,5 and HR values (Polar heart rate monitor,

Polar etc) were collected at the end of each six minute block. All women were given

the opportunity to increase or decrease treadmill speed at the end of each six-minute

interval to ensure replication of a self-selected exercise pace. Speed was increased or

decreased by a technician when requested and any change in speed noted. All women

were familiarised with the exercise environment, equipment and the measurement

scales to be used, following the rest period, and before sampling began for their

exercise session. Throughout the exercise session, participants were verbally

reminded to continue exercising at their normal pace.

Energy expenditure (EE) was calculated using the energetic equivalent of 20.1 J.mL-1

O24 from the indirect calorimetry values collected at rest and at the end of each six

minute block. Resting EE was calculated from the mean oxygen consumption ( &VO2)

over the two minute sampling period immediately prior to the exercise session, while

the 60 seconds of breath analysis was averaged and used to estimate EE per minute

during the exercise session at minutes 6, 12, 18, 24 and 30. The average EE for the


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treadmill session does not include resting values, thus these data represent EE

associated with activity only.

To find the mass-specific net cost of walking (Cw), ie the energy cost of moving 1 kg

of body mass 1 metre, the energetic equivalent of the average rate of oxygen

consumption at rest and over the 30 minute treadmill walk was calculated using the

energetic equivalent of O24 and expressed in J.sec

-1. Only participants with a

respiratory exchange ratio (RER) < 1.0 were used in these analyses to ensure aerobic

metabolism was the primary metabolic pathway.28

Two women in the YOU group

had RER values > 1.0 and were excluded from further analyses; thus, the final

participant number for the YOU group was eighteen. The mass-specific net metabolic

rate was calculated by subtracting the resting EE from the average EE over the 30

minute TM walk, and dividing by body mass (W.kg-1

). Mass specific net metabolic

rate was then divided by the average treadmill speed (metres.sec-1

) to give the net

metabolic cost of walking (Cw, in J.kg-1

.m-1

).2, 22

Metabolic equivalents were calculated to facilitate direct comparisons of exercise

intensity between the self-selected walking pace of our groups and the recent ACSM

recommendations defining moderate intensity as activities using 3.0-6.0 METs.14

For

the purposes of this estimation, resting metabolic rate was assumed to be 3.5 ml.kg-

1.min

-1.

Maximal oxygen consumption ( &VO2max) test

Following the completion of the walk, a maximal oxygen consumption exercise

( &VO2max) test was undertaken on a treadmill using individualised ramp protocols, with


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predicted &VO2max and grade increments calculated from established equations.9

Treadmill speed was determined from the average of velocities chosen during the 30

minute walking exercise session and was held constant throughout the test. Following

a two minute warm up, grade was increased every minute until volitional fatigue.

Grade increments were calculated using predicted &VO2max estimated from equations

of Cooper and Storer (2001).9

Continuous breath analysis (Sensormedics 2900 metabolic cart, Sensormedics,

California, USA) and heart rate (HR) were undertaken throughout the exercise test

and values were recorded at 20 second intervals.

Statistical Analysis

Group mean ± standard error of the mean (SEM) was used to describe the data.

Shapiro-Wilk testing of all variables was undertaken to ensure normal distribution of

data; all data were found to be normally distributed. Independent t-tests were

performed to determine differences in physiological parameters between the YOU and

OLD groups. To control for self-selected treadmill speed, analysis of covariance

(ANCOVA) was used to examine the influence of group (YOU or OLD) on the net

energetic cost of walking (J.kg-1

.m-1

). A stepwise multiple regression analysis was

undertaken using combined group data (n=38) to determine the predictor(s) of

treadmill speed selection (m.sec-1

). Independent variables entered into the model

were age, height, weight, and &VO2max (expressed as ml.kg-1

.min-1

). Pearson product-

moment correlation coefficient testing was undertaken to investigate the relationships

between the independent variables and walking speed. Statistical Package for the


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Social Sciences (SPSS) was used for all statistical analyses (SPSS Inc, v14.0,

Chicago, IL). Significance was set at p

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variable accounted for 61% of the variance in walking speed (β coefficient = 0.78;

p=0.0005). Age was significantly (p

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slower in the oldest group of adults (80 years), with oxygen cost per distance walked

approximately 22% higher than those aged 25 years. Waters et al (1983)29

reported

that normal walking speed was approximately 10% slower in older (60-80 years)

versus younger (20-59 years) adults, although gross oxygen cost at this speed was

similar. However, when expressed per distance walked, oxygen costs were 8% higher

in the older group. Similarly, when treadmill speeds are fixed, energy costs are also

higher for older individuals. Walking at speeds over a range of 0.7-1.8 m/s, average

metabolic costs of walking have been reported as 31%22

and 20%25

higher in older

adults when compared with younger adults. In the present study, self-selected

walking speed at exercise pace was approximately 23% lower in our OLD group than

in the YOU group, while the average net Cw was 15% higher. Our results are similar

to those found by Haveman-Nies et al. (1996)15

and Malatesta et al. (2003)19

for

‘normal’ walking pace, while our finding of a similar gross rate of oxygen

consumption between the two groups, despite significantly slower walking speeds

selected by the older women, concur with those of Waters et al. (1983).29

Furthermore, the rate of energy consumption-speed relationship in our women falls

within the range of normative data for adults aged 20-80 years produced by Waters et

al. (1988).29

The discrepancies amongst studies investigating speed and metabolic

costs of walking are likely to be attributable to methodological differences.

Participant age groups vary, which may distort speed-related results as the most rapid

decline in walking speed occurs at age 62 years.16

Gender may also have an effect,

as gait speed and stride length and frequency have been shown to differ between

males and females.29

The modality used to assess gait speed may also explain part of

the differences. Some studies used subjective instruction to assess walking over a

range of speeds on level ground or treadmill,15, 26

while others used fixed speeds while


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participants walk on a treadmill.19, 21

Furthermore, oxygen consumption values that

have not been corrected for resting oxygen consumption may underestimate the

metabolic costs attributable to locomotion.

We also demonstrated that when walking speed is held constant, the cost of walking

was increased by was 0.30 J.kg-1

.m-1

in the older women. This would equate to the

expenditure of an additional 111.3 kJ in one hour of walking at the same pace as the

YOU group. While this level of energy expenditure appears small, in older individuals

who may be under-nourished, the increased energy requirements for exercise may

further deplete existing limited energy reserves.

Older individuals are being encouraged to undertake regular physical activity;

however, the impact of increased daily exercise energy expenditure on activities of

daily living (ADL) has not been fully appreciated. In a free-living situation, Harris et

al.,(2007)13

using a telemetry-based system, calculated that older individuals walk

approximately 6.5 miles (10.4 km) per day, three miles (5 km) per day less than

younger individuals and, in agreement with the results of the present study, energy

expenditure per mile walked at the same speed was higher in the older group.13

In

addition to a reduction in total distance walked at a higher energy cost, Harris et al.

(2007) observed a decline in daily nonexercise movement, standing time and

movement accelerations in the older individuals, although total energy expenditure

did not differ between the young and old individuals.13

Thus, it could be proposed that

some compensatory behaviours are undertaken by older individuals to limit energy

expenditure of ADL throughout the day. Careful attention to nutritional intake and

recovery strategies should be incorporated when programming exercise for older


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individuals. Failure to do so may result in an even greater reduction in movement

associated with ADL.

Cardiorespiratory demands are increased in older adults walking at their preferred

speed, evidenced by the higher relative fractions of &VO2max and ventilatory threshold

used compared with younger individuals.20

In the present study, while differences

were not significant, the older women were walking at a slightly higher percentage of

their maximum aerobic capacity than the younger group, 47% vs 44%, respectively.

This compares well with a study by Malatesta et al. (2004)20

who found preferred

walking speed in a group of 65-year old adults to be undertaken at 43% &VO2max,

however; the women in our study were asked to walk at their ‘normal exercise pace’

and thus the working fraction might have been expected to be somewhat higher.

Despite this, the self-selected exercise pace in both groups satisfied the recent ACSM

physical activity definition of ‘moderate intensity’ (3.0-6.0 METs), which is

recommended for cardiovascular maintenance in healthy adults aged 18-65 years.14

A consistent finding in previous studies is that walking speed declines with age.8, 10, 15,

16, 19-21, 29 Reasons proposed for this have been attributed to age-related changes in

physiological function and biomechanical efficiency.10, 15, 16, 20-22, 26, 27

We found

associations between self-selected exercise pace and age, height, and maximal &VO2;

which concur with those previously reported.16, 20, 27

In agreement with studies by

others,10, 20

we found maximal oxygen consumption, expressed relative to

bodyweight, to be the primary determinant of walking speed when our groups were

combined, accounting for 61% of the variance. Malatesta et al. (2004)20

reported 48%

of the variance in walking speed in adults aged 65-80 years to be explained by the


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fraction of &VO2 corresponding to the ventilatory threshold at preferred walking speed.

Given the numerous studies which have reported a decrease in walking speed with

advancing age, it is somewhat surprising that, as with our study, Malatesta et al.

(2004)20

and Cunningham et al. (1982)10

also did not find age to be an independent

factor in determining preferred walking speed. In both studies, and with ours, the lack

of an independent age effect on walking speed may be explained by the strength of

association with the use of more stringent statistical procedures. Simple correlational

analyses in all studies reveal significant negative associations between age and

aerobic fitness and walking speed; however, age effects are lost in multiple regression

analysis.10, 20

It is also notable that the self-selected speed of walking for exercise in

our two groups of women coincided with the normal, or preferred, walking speed of

individuals in similar age categories reported elsewhere. 10, 16, 19, 26, 27, 29

In summary, this study demonstrated that the relative energetic costs of walking at a

self-selected exercise pace over 30 minutes are significantly higher in older women

when compared with younger women. Furthermore, preferred walking speed is

determined by the individual’s maximal aerobic fitness level. Some effort should be

made to assess initial fitness levels in older women and the higher relative energy

costs of walking kept in mind to ensure an exercise plan is developed that will be safe

and effective for that individual.


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Acknowledgments

Funding was provided by a University of Otago Research Grant. We would like to

thank Vicky Phillips and Kate Parrott who collected the data for this study.


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Table 1. Demographic and baseline physiological data for younger (YOU) and older

(OLD) groups (mean ± SEM)

YOU

(n=18)

OLD

(n=20)

Significance

(p value)

Age (years)

40 ± 1

59 ± 2

0.001

Height (cm)

165 ± 0.02

161 ± 0.01

0.1

Weight (kg)

74.0 ± 2.9

70.0 ± 2.3

0.3

BMI (kg.m2)

27.1 ± 1.1

26.8 ± 0.9

0.8

&VO2max (L.min-1

)

2.42 ± 0.07

1.82 ± 0.09

0.001

&VO2max (ml.kg-1

.min-1

)

33.37 ± 1.25

26.55 ± 1.39

0.001

HRmax (bpm)

179 ± 2

163 ± 2

0.001

BMI = body mass index; HR = heart rate; bpm = beats per minute


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Table 2. Physiological data from the treadmill walking (TM) exercise session for the

younger (YOU) and older (OLD) groups (mean ± SEM)

YOU

(n=18)

OLD

(n=20)

Mean TM speed (m.sec-1

)

1.47 ± 0.05

1.14 ± 0.07**

Net Cw (J.kg-1

.m-1

)

2.39 ± 0.07

2.74 ± 0.10*

Mean &VO2 (ml.kg-1

.min-1

)

14.0 ± 0.8

12.2 ± 0.7

Mean HR (bpm)

110 ± 3

104 ± 3

METs

4.0 ± 0.2

3.5 ± 0.2

Mean RPE

11.4 ± 0.2

11.1 ± 0.2

Cw = energetic cost of walking; HR = heart rate; METs = metabolic equivalents; RPE

= rating of perceived exertion. *

p

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Table 3. Pearson correlation coefficients between predictor variables and self-

selected walking speed (n=38)

Independent variables

Walking speed (r)

Age

-0.61**

Height

0.39*

Weight

-0.96

&VO2max (ml.kg-1

.min-1

)

0.78**

* p

Energy costs of walking

1.5

2

2.5

3

3.5

4

0.8 1.0 1.2 1.4 1.6 1.8 2.0

Treadmill Speed (m/sec)

Cw (J/kg/m)

FIGURE 1. Cost of walking in relation to self-selected treadmill speed for YOU

(r2=0.10, p=0.16; slope coefficient ± standard error, 0.528 ± 0.359; solid line, open

triangles) and OLD (r2=0.01, p=0.59; 0.190 ± 0.350; dashed line, solid squares). The

slopes of the lines did not differ significantly (p=0.53).


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